Telephone lines have traditionally been developed and installed in networks for transmitting voice communications. More recently, they have found a use in transmitting digital type data, by means of modulator-demodulators generally known as modems. Such telephone links have transmission characteristics which are well adapted to transmitting analog signals at frequencies of not more than about 4 kHz. When they are used for transmitting digital signals, they tend to be useable at data rates of up to about 4800 bauds. Such performance limits the volume of information which may be transmitted in given time.
Thus, while telephone lines are well adapted to transmitting human speech or to transmitting computer messages in the form of relatively low data rate binary signals, they are not suitable for real time transmission of images which are liable to change quickly in time. Image transmission requires a relatively large amount of information to be transferred in comparison to the volume of information needed for a voice message. The image must be divided into point-like zones or "pixels" which are smaller with increasing definition to be transmitted. A signal representative of the brightness of each pixel must be produced. The set of such signals must then be transmitted, generally in series in the form of a sequence. Each sequence corresponds to an image, and the level of the signal at a given instant in the sequence corresponds to the brightness of a corresponding point in the transmitted image.
Such sequences are transmitted, as is well known, in television systems. In order to show moving scenes in such systems, the transmitted images must be renewed at a relatively high rate. It is necessary to have links capable of transmitting a frequency band whose width is orders of magnitude greater than the bandwidth of single telephone links.
Attempts have already been made to improve the comfort of telephone communications by transmitting the parties' images to each other by means of such a TV-like system which is generally called a visiphone. The development of such systems has been held back by their cost which is particularly related to the fact that the information transmission capacity required for transmitting adequate images is twenty to forty times greater than the capacity needed for a speech only telephone line.
When the images to be transmitted are relatively static, proposals have already been made for reducing the data transmission rate. One such system is described in a paper entitled "Video teleconferencing at 9600 bauds" by Robert H. Wallis and William K. Pratt in the "IEEE Picture Coding Symposium, Montreal 1981. An opto-electronic cine-camera is used to produce a transmissible image of a subject such as the head and shoulders of a speaker in a remote conference. The resulting electrical signals are converted into binary in such a manner that each "pixel" is represented by a signal capable of taking only two values. The resulting sequence obtained for the image is then subjected to a coding process capable of compressing the information content in the sequence so that it may be transmitted over a telematics line at a rate of about 9,600 bauds. Images obtained by this procedure are of relatively poor quality, even when the image renewal rate is limited to only a few images per second. This technique is thus not suitable for real time re-transmission of scenes in which movement must be produced to some degree of accuracy.